1. Field of the Invention
This invention relates to a transducer and more particularly to an acoustic matching member therefor.
2. Discussion of the Background
There are a number of useful measurement applications that are conveniently achieved by sending and receiving ultrasonic signals in gases in the frequency range between 100 KHz and 1 MHz or above. At these high frequencies, the conventional construction of sound transducers employed at lower frequencies (e.g. audio frequencies) is impractical as the overall dimensions become very small.
The normal method of making high frequency ultrasonic transducers is to use a selected piece of piezo ceramic (e.g. lead zirconate titanate or PZT) resonant at the required frequency. PZT is a hard, dense material of high acoustic impedance (approximately 3.times.10.sup.7 in MKS units), while gases have very low acoustic impedance (of the order of 400 in the same units). PZT on its own gives very poor electro acoustic efficiency due to the large acoustic mismatch, even though this is improved somewhat by resonant operation.
Typically, the piezo ceramic element is a cylinder, whose circular end faces move in a piston-like manner in response to electrical stimulation of electrodes applied to these faces. The normal method for reducing the acoustic mismatch to gases is to apply an acoustic matching layer to the selected operational face of the PZT disc. This layer is a material of relatively low acoustic impedance whose thickness is one quarter of an acoustic wave length in the material at the chosen frequency of operation. This dimension results in a resonant action whereby (for sending) the small movements obtained at the face of the PZT cylinder are magnified considerably, and acceptable (though still now high) efficiency can be obtained. The criteria for acoustic-electric conversion (i.e. receiving) are the same as for electro-acoustic conversion (i.e. sending) and the same transducer may be used for both.
The efficiency attainable by this technique is limited entirely by the characteristics of available materials. An ideal material would have an acoustic impedance on the order of 10.sup.5 and very low internal losses, and also must be stable, repeatable and practical for use. There are no hitherto known materials that meet all these criteria. Some common approximations to the ideal requirements are:
1. Silicone elastomers. This class of materials is commonly used and provides a useful performance in many applications. Acoustic losses are low. Acoustic impedances down to about 7.times.10.sup.5 can be attained. A significant drawback with these materials is a large variation of acoustic wavelength with temperature (typically 0.3%/K). This factor limits the range of operating temperatures over which correct reasonant matching is obtained.
2. Polymers generally. Many polymers give useful performance. Acoustic impedance is higher than for silicones--down to 1.5.times.10.sup.6 so overall efficiencies are lower, but reasonably stable materials can be found.
3. Liquids and gases. Examples in the literature may be found of the experimental use of multiple acoustic matching layers. Liquids have generally very low losses and acoustic impedances down to about 10.sup.6. If a gas is compressed, its acoustic impedance rises directly with the compression ratio, and a captive volume of liquid or highly compressed, dense gas may be used as an acoustic matching layer. Such techniques are not practical for commercial application.